I hate to ask this but how do you know when the black tank is full?? Also our Caravels' door lock came with the lock core not in the knob however we do have the core, is that an easy fix??(We would like to lock it up when we are not in it) thanks in advance!!!

On mine, when the tank is empty, step on the flush petal, the flap opens, and you can see into the tank. Shine a flashlight down the hole and you can see the bottom of the tank. As it fills you can follow the same procedure and see the level rise. I can't really tell the exact level, but it does give me an idea. I'm am not sure if you can see into the tank if you have a more modern toilet.